Radiotherapy involves subjecting a target, such as a tumor, within a patient to one or more radiation beams. Ideally, a specific dose should be delivered to the target and minimal radiation should reach the surrounding tissue. In particular, the radiation to critical tissues or organs, such as the heart, should be minimized. Normally the radiotherapy is distributed in a number of sessions, for example once a day for a number of days or weeks.
For planning the radiotherapy overall, a 3D planning image of the relevant part of the patient is generated and segmented. Segmenting the image means determining the boundaries of various regions of interest, such as the target and surrounding tissue, organs etc. and entering these boundaries into the image. The planning image may also be based on an image from an image atlas. Since the size and shape of a tumor, and other organs, and their location, will usually change over the course of the radiotherapy, it is common practice to take a fraction image at one or more times during the radiotherapy, for example, at the start of each radiotherapy session. Information from the fraction images is used to update the radiotherapy treatment plan. This is referred to as adaptive radiotherapy treatment planning.
Dose planning requires information about the location of the various organs and also about their material properties, such as density and/or atomic composition. Density information is used for dose planning. If photon radiotherapy is used, the density and atomic composition determine the attenuation of the radiation. If ion radiotherapy such as proton radiotherapy is used, the density and atomic composition determine the stopping power, which affects the distance that the ions will travel within the patient's body. For the initial planning, this information is taken from the planning image. The fraction images are typically used to determine the new boundaries of the regions of interest, to aim the radiation beams correctly.
Therefore, the planning image should comprise information not only about the contours but also about the material properties of each region of interest. As the geometry of the tumor and other tissues changes during the course of therapy, the fraction images are used to obtain up-to-date contour information. Therefore, the fraction images may have considerably less information than the planning images. This is advantageous because it allows for a lower radiation dose to the patient for each fraction image and also for the use of less expensive imaging systems for taking the fraction images. For example a fan beam CT scan (referred to in this document as CT) may be used for the planning image while Cone Beam CT (CBCT) scans are used for the treatment images. CT images comprise all the information needed for dose planning but are relatively expensive and involve a higher radiation dose to the patient than for example CBCT. CBCT on the other hand does not always provide reliable information about material properties and in particular is subject to distortion such as cupping distortion, where the intensity of the image is misrepresented near the edges of the image. Other imaging technologies involve even less or no radiation but do not provide all the information necessary for proper treatment planning.
In adaptive treatment planning, the process of establishing a mapping of coordinates between the planning image and one or more fraction images is called image registration. Several registration methods are known in the art. Particularly preferred are deformable image registration methods, which take into account both the individual motion of each organ, and the deformation that typically occurs during radiotherapy. In deformable registration, each voxel in the planning image is mapped to a corresponding voxel in the fraction image, so that the movement of the corresponding part of the body can be determined. The terms deformable registration and elastic registration are used interchangeably.
European Patent application EP 1 778 353 discloses the use of deformable registration for adapting a radiotherapy plan between the treatment sessions. Contour information is obtained in the fraction images by utilizing deformable registration with the planning image and used to update the radiotherapy plan.
For most of the regions of interest the density varies very little over the course of the radiotherapy. For others, the density may vary significantly within short time frames. For example the size and content of the bowel and the urinary bladder will vary with time. The presence of air or liquid will affect the density, and therefore the attenuation or stopping power of the organ, significantly. Currently available methods for image registration do not account for these changes.